(1) Field of the Invention
In general, the present invention is linked to measurement techniques of functional parameters in combustion internal engines but, more specifically it is related to the signal processing method in order to implement a non intrusive tachometer for turbo-compressors and the measurement device used.
(2) Prior Art
U.S. Pat. No. 4,573,358 is a “device to detect vibrations in turbines blades”. It consists of a plurality of sensors evenly distributed over a series of blades coupled to the periphery of a turbine disk. The sensors are able to provide a signal at the exit in answer to the passing of the blades. One of the blades is selected for monitoring to indicate the degree of relative location to an -x- reference at a certain distance constituting a small window. Thus, when the blade selected passes by a sensor its proper exit signal conditioned to form a pulse is registered during a period of time corresponding to the window. The sequential pulses of the sensors form a train of pulses modulated by each vibration of the blade. It has an FM sensor to demodulate the train of pulses and to derive the vibration information.
When analyzing this patent, an inconvenience arises: it can only discriminate the harmonic vibrations to half the number of sensors. Bigger harmonics are also detected but they appear as responses to smaller harmonics. This phenomenon of showing a bigger frequency harmonic as a smaller one is known as “aliasing”: difficulty in the precision of conclusions.
On the other hand in this U.S. Pat. No. 4,573,358, sensors of proximity permanently installed in the machine are used. The plurality of sensors is spread in the periphery of the selected blades line. It can monitor a blade at any time, but it has the following problem: there are technical difficulties to install and mount the plurality of sensors needed inside the turbine. Besides it cannot separate the axis torsion vibrations and the asynchronous vibrations of the individual blades.
In U.S. Pat. No. 4,887,468, “a device used to determine vibrations in blades applicable to asynchronous turbines” is described. It describes the use of a maximum of two sensors mounted in circumference along a line of blades to detect the real time of arrival of the blades to the sensors. The estimated times of arrival of the blades to the sensors are determined and compared to those times measured in order to obtain the data deflexion referred to the blades. Then the harmonic analysis to determine the level of vibration of each non integral harmonic is performed.
It turns that in this patent Fourier's conventional analysis should be applied to determine the amplitude of the components of the vibration to each frequency. It happens that the system detects the presence of synchronous vibrations. Then obtaining only two tests doesn't provide enough information to discriminate among several integral harmonics.
U.S. Pat. No. 5,206,816 is “a system and method for monitoring synchronous vibrations of blades in turbo machines”. At least three sensors able to detect the arrival of each blade, regarding the cylinder, are arranged around their tips. The location of the sensors is selected to minimize error influence on the data. To make the installation of the sensors inside the cylinder simpler, they can be displaced non-uniformly. This location has the purpose of building Fourier's transformations using the real times of arrival of each blade to each sensor expressed as a fraction of a revolution of the rotor. The transformations matrix is built for a predetermined discreet group of M harmonics so that they are analyzed simultaneously. The number of harmonics that can be simultaneously analyzed is relative to the number of sensors used. Information on the time of arrival of a blade to each sensor is expressed as a fraction of a revolution of the rotor. It is analyzed to determine the amplitude of the vibration of the blade of the harmonics selected by the group. Calculations are repeated for a group of harmonics to interpret the result exactly, in the event that a significant vibration of a harmonic not included in the initial group arises.
U.S. Pat. No. 5,479,825 “system of microwaves for monitoring vibrations in blades of turbines by means of the continuous transmission of energy waves in microwaves”.
U.S. Pat. No. 5,483,833, refers to “a method and apparatus for monitoring aircraft components”. Operability of aircraft mechanical components is monitored by analyzing the voltage output of an electrical generator of the aircraft. Alternative generators, for a turbine-driven rotor aircraft, include the gas producer turbine tachometer generator, the power turbine tachometer generator, and the aircraft systems power producing starter/generator. Changes in the peak amplitudes of the fundamental frequency and its harmonics are correlated to changes in condition of the mechanical components.
U.S. Pat. No. 6,584,949 refers to the “vibration analysis of rotating blades” of the gas turbine type. It describes a method that includes the steps for providing a strain gage on one of the blades and that provides either one or two test tips located in a space around the blades. The strain gage data allow the vibration of a single blade to be completely characterized, while the data from the test tips allow the amplitude or speed of all the blades to be determined. The relationship between the data from the test points for a single blade and the data from the strain gage can be established. This relationship can be assumed to apply to all the blades and in this way the strain induced by the vibration will be determined in all blades. Because of its own characteristics the previous work doesn't interfere with the present invention.
Therefore all these works being analyzed, it arises that they use dispositions that do not interfere with the present work.